Thanks to their excellent mechanical strength, moldability, transparency, recyclability of resources, etc. polyester resin containers, such as PET bottles, have been recognized as containers for food and drink and they have been in great demand. In particular, recently polyester resin containers have been habitually used by consumers as portable small containers for beverage use. The heat resistance and pressure resistance of such small containers for beverage use have been remarkably improved by the development of the two-stage blow molding process (refer to Japanese Patent Publication No. 4-56734 (claim 1 and the upper section of the left column on page 2)) and small containers have become capable of being used for high-temperature beverages and beverages requiring high-temperature pasteurization. Thus, they have become able to meet consumers' strong requests for portable high-temperature beverages for the winter season in daily life.
Furthermore, reuse systems of PET bottles have been established to meet the social requests for resource savings and environmental protection, and the cleanliness due to the transparency of containers and the sense of safety by seeing the beverages inside through containers meet consumers' cleanliness-oriented trends, with the result that the demand for polyester resin containers represented by PET bottles has been increasing further.
Recent consumers prefer beverage bottles of flat shape having a rectangular section etc. because these bottles are easy to hold and beautiful thanks to their complex shapes, whereas bottles having a circular section tend to be avoided because they are difficult to grip due to slipperiness when their surfaces become wet and because they lack aesthetic impressions due to the simple shape of a circle.
Flat containers having flat shapes with rectangular, elliptical or other sections that tend to be liked by consumers and have a very strong demand are manufactured by blow molding (molding by blowing the air), with a preformed, bottomed parison (a preform) inserted into the interior of a mold having a flat section and attached thereto. On the occasion of molding into a flat shape, the wall thickness of a container wall tends to become nonuniform due to a difference in the amount of elongation in the major axis direction and minor axis direction of the section of the container and for other reasons.
As a measure to prevent this, there is known, for example a method for obtaining a flat container that involves inflating a bottomed parison having a circular section in a first blow mold into an intermediate molded article, inserting this intermediate molded article into a second blow mold, flattening the intermediate molded article into an elliptical shape by mold closing, and performing thereafter blow molding (refer to Japanese Patent Publication No. 59-53861 (Claims for the Patent and FIG. 1)). In this method, however, the shapability by blow molding is insufficient and wrinkles due to the waviness of concavities occurring partially on the surface of an intermediate molded article are apt to remain on the surface of a product, posing the problems of a poor appearance and insufficient strength.
In addition, some methods and apparatus for aiming at a uniform wall thickness in the manufacture of a flat container by blow molding have been disclosed. They are, for example, a bottomed cold parison blow molding process, which involves heating a bottomed cold parison before blow molding so that a portion stretched in the minor axis direction of a flat shape obtains a higher temperature than a portion stretched in a major axis direction or using a bottomed parison that is formed with thickness uniformity so that the wall thickness of a portion stretched in the major axis direction becomes large and the wall thickness of a portion stretched in the minor axis direction becomes small, and radiant heating the bottomed parison in its periphery while rotating the bottomed parison in the axial direction (refer to Japanese Patent Laid-Open No. 2000-127230 (Claims for the Patent and paragraphs 0005 to 0008)), and a molding mold that performs blow molding by housing a parison within a cavity of a split mold, advancing moving mold member to the parison surface and flattening the parison, and molds a container having high flatness which is such that the size ratio of a flat surface to side surfaces is not more than ½ with a uniform wall thickness (refer to Japanese Patent Laid-Open No. 8-294958 (Claims for the Patent and paragraphs 0003 and 0004)). In general, however, it is difficult to obtain the uniformity of the wall thickness of container walls because the elongation and expansion of a parison within a cavity do not become uniform due to the flat section and material accumulation occurs due to insufficient elongation on the minor axis side, with the result that it is difficult to manufacture a flat container with a uniform wall thickness of the container body.
When the wall thickness becomes nonuniform, the mechanical strength and heat resistance of a container decrease due to thin-walled portions. When such containers are used as high-temperature beverage containers, there is a fear that the deformation of the containers may occur because they cannot withstand inner pressure loads due to the expansion of beverages in the containers at high temperatures and external pressure loads due to the shrinkage of the interior during temperature drops.
On the other hand, in a flat container obtained by blow molding, from viewpoints different from the intension to improve the blow molding process, there has scarcely been made any proposal as yet to make improvements on mechanical strength, heat resistance, etc. specific to flat containers by specifying flat containers by physical properties and imparting characteristics to flat containers and to prevent the fear that the deformation of the containers may occur because they cannot withstand inner pressure loads due to the expansion of beverages in the containers at high temperatures and external pressure loads due to the shrinkage of the interior during temperature drops, and under the present circumstance, there is only a proposal which is such that a polyolefin-based flat container obtained by injection blow molding in which only the flatness ratio (the maximum major axis of the body/the minimum minor axis of the body) and wall thickness are specified is shown (refer to Japanese Patent Laid-Open No. 11-170344 (claims 1 to 3 of Claims for the Patent)).
In view of the condition of such conventional techniques in flat thermoplastic resin containers such as polyester resin containers obtained by blow molding, which are liked by consumers as beverage plastic containers and their demand is especially increasing because they are easy to carry and beautiful thanks to complex shapes, the present inventors consider that the first problem to be solved by the invention is to realize a flat container excellent in mechanical properties and heat resistance by specifying flat containers by physical properties and imparting characteristics to flat containers and improving mechanical strength, heat resistance, etc. specific to flat containers. At the same time, in the manufacture of a plastic flat container for beverage having a flat sectional shape, such as a rectangle by blow molding, from the standpoint of a molding method of such a flat container, the present inventors consider that the second problem to be solved by the invention is to realize a molding method for easily manufacturing a flat container with an improved uniformity of wall thickness, with improved mechanical strength, heat resistance, etc. and with a good appearance without a wrinkle.